1. Field of the Invention
Embodiments of the present invention relate to a pressure balancing device, such as for a transducer.
2. Description of the Prior Art
Downhole logging tools are frequently designed to be pressure balanced. This is where a measurement sensor is located within the body of the tool and is protected from well fluid by appropriate sealed housings, but where some form of pressure transmission device is used to compress a fluid within the tool to approximately the same pressure as the well fluid outside. In this manner, the sensor is exposed to the well pressure, without suffering the consequences of being immersed directly in the well fluid. The compensating mechanism must also allow for the internal fluid to expand at low pressure but high temperature.
Pressure compensating mechanisms regularly used downhole include pistons, rubber bladders and flexible metal bellows. Each mechanism has advantages and disadvantages in terms of the resulting pressure difference between the inside and outside of the tool, such as maintainability, cost, durability and resistance to attack from the chemicals in the well. Of these methods, compensating pistons are often used, due to their simple design and rugged construction. However, inherent in the design of a piston compensating system is the need for dynamic sliding seals, which experience high hydrostatic pressures and therefore suffer from high friction, and resist the motion of the piston. This resistance to motion results in a differential pressure across the piston, which corresponds to a minimum force required to move the piston against its seals. The higher the seal friction, the larger the differential pressure will be between the inside and the outside of the tool. In cases where the sensor inside the tool needs to be at the same pressure as the well fluid for accurate measurement, this differential pressure results in measurement error and is undesirable. In a similar manner both the bladder and the flexible metal bellows have stiffness in their structures, which resist motion and hence, generate a differential pressure.
A typical example of a downhole pressure sensitive device is a cable head load measuring tool. This tool is placed at the top of a logging tool string and is designed to report to the user the force being exerted on the tool string by the wireline or tractoring tool. Essentially, the tool must measure differential axial loading in the string, but not the axial load produced solely by the well pressure surrounding the string. Using a pressure balanced load measuring sensor is one solution, which inherently requires a good pressure compensating device.